This invention relates to improving the stability of a catalyst used to lower the cloud or turbidity point and the filterability limit temperature of gas oils.
It is already known, according to J. E. Marechal (VIth Petroleum Conference Section III paper 1 PD 7) that pouring problems arising from cold temperature resistance of petroleum cuts, and especially gas oils, are bound to the content of long chain linear paraffins (heavy n-paraffins) in said cuts. The aim of the invention is to treat hydrocarbon petroleum cuts containing paraffins, or, being themselves pure or mixed paraffins.
Straight run distillation gas oils contain a high proportion of C.sub.12 -C.sub.23 n-paraffins. The heaviest cuts can contain longer C.sub.12 -C.sub.30 paraffin chains. These hydrocarbon petroleum cuts can be gas oils whose initial distillation point is generally at least equal to 150.degree. C. and whose final distillation point, usually set at 455.degree. C., may reach 530.degree. C. when the cuts are obtained by distillation under vacuum.
In the latter case, lubricants can be obtained from these cuts, provided a previous treatment is carried out for eliminating the aromatic compounds by hydrorefining or solvent stripping. The problem thus consists in lowering the pour point of the product obtained, without significantly decreasing its viscosity index while ensuring a yield suitable for the operation. Pour point of lubricants is defined by AFNOR 60105 standard. As regards the gas oils, these must, in order to satisfy commercial quality requirements, fulfill engine gas oils and domestic fuel specifications. Bearing this in mind, it must be noted that the most restrictive characteristics are the sulfur content and pour characteristics.
The pour characteristics most widely used for gas-oils are the pour point and turbidity point (T.P.) defined according to AFNOR T 60105 standard and the filterability limit temperature (FLT) defined according to AFNOR N 07042 standard.
Various treatments can be carried out so that straight run gas-oil cuts comply with said specifications.
When the sulfur content is too high, it is usual to carry out a desulfuration treatment in the presence of hydrogen. This treatment is a catalytic hydrodesulfuration (HDS) obtained by contact with a Cobalt-Molybdenum type catalyst supported on alumina. But this type of treatment does not enable the pour properties of treated petroleum cuts to be improved.
Two solutions are therefore possible:
the first solution, the efficiency of which is however limited, consists in the use of additives; PA1 the second consists in proceeding with a catalytic hydrotreatment called dewaxing or hydroconversion. PA1 catalysed hydrogenolysis reactions of C-S links by metallic sulfides; PA1 hydrocracking and hydroisomerisation reactions of paraffins occuring on contact with the acid sites of the catalyst; PA1 reactions--alas, inevitable--of coke formation (polymerisation, cyclisation, condensation) on the acid support. PA1 a hydrodesulfuration catalyst A constituted by a metal from Group VI and a metal from Group VIII of the Periodic Table placed on a non-acid or weak acidity oxide such as pure alumina containing not more than 6% by weight of silica; PA1 a hydroconversion catalyst B constituted by a metal from Group VI and a metal from Group VIII of the Periodic Table on a synthetic or natural silica-alumina type support having a certain acidity, e.g. a silica-alumina support having a silica content higher than 5% by weight.
The catalysts used in certain hydroconversion processes are constituted by cobalt or molybdenum metallic sulfides, for example, on an acid support which thus no longer only consists in alumina. These processes involve parallel desulfuration and hydroconversion reactions. The reaction mechanism of this type of catalyst causes three types of reaction to occur:
It is this second type of reaction which causes the improvement of the cold temperature pour properties of petroleum cuts by conversion of n-paraffins.
As regards the third type of reaction, it results in a poor stability catalyst with time which manifests itself as a rapid decrease in the speed of the principal reactions thus making it necessary to rapidly increase the temperature of the reactor.